PROJECT SUMMARY/ABSTRACT In the United States, the incidence of urinary stone disease (USD) is rapidly increasing, affecting 8.8% of the population in 2010, up from 2.6% in 1968, with half of patients exhibiting recurrent episodes. Calcium oxalate stones comprise approximately 80% of USD cases and oxalate metabolism is provided exclusively by gut bacteria. The canonical hypothesis for the increase in USD incidence is that a single oxalate-degrading species, Oxalobacter formigenes, is sufficient to reduce urinary oxalate excretion enough to prevent calcium oxalate stone formation and is lost due to oral antibiotic use. However, in a paradigm-shifting series of studies, we have found that 1) O. formigenes is neither necessary nor sufficient to reduce urinary oxalate excretion or prevent USD; 2) Oral antibiotics increase the risk of USD regardless of their effect on O. formigenes; 3) A functionally diverse oxalate-degrading microbial network (ODMN) is responsible for oxalate metabolism rather than a single species; 4) The ODMN prevents the toxic effects of exogenous oxalate on the host and the microbiome; and 5) The ODMN is negatively associated with oral antibiotics in both animal models and in a clinical cohort of USD patients. Given these new data, it is critical to further understand how the ODMN maintains a homeostatic environment relative to oxalate and contributes to the prevention of USD. The objective of the current proposal is to establish the host-microbe mechanisms through which the ODMN facilitates persistent oxalate metabolism in vivo to prevent enteric hyperoxaluria. Oxalate can either stimulate or inhibit gut microbial diversity depending on the baseline composition. Our preliminary work shows that when laboratory rodents with a reduced ODMN are exposed to exogenous oxalate, urinary oxalate excretion gradually increases over time, there is a loss of microbial diversity, and signs increased gut permeability are apparent. These effects are reversed in animals with a robust ODMN. The by-products of oxalate metabolism are formate and CO2. Formate has known toxic effects for both humans and bacteria. However, acetogenic, methanogenic, and sulfate-reducing bacteria (AMS), which can use formate and CO2 as carbon and energy sources, are consistently present in the ODMN. Thus, we propose oxalate-degrading and AMS bacteria exhibit metabolic interactions that synergistically promote persistent colonization and reduce inflammation of host intestinal epithelium, thereby reducing oxalate absorption. To test this hypothesis, we will Investigate the mechanistic microbe-microbe and host-microbe interactions that modulate enteric hyperoxaluria, determine the role of AMS bacteria in preventing hyperoxaluria, and investigate the mechanisms that drive the loss of ODMN function in a clinical cohort of patients.